The present disclosure relates generally to a structure facilitating electrical contact between devices such as printed circuit boards and integrated circuits, referred to generally as an interposer, and more particularly to an interposer which utilizes microsprings as electrical contacts.
Electronic apparatus such as portable computers, cell phones, etc. have reduced in size from generation to generation. This reduction in size has resulted in smaller and smaller discrete devices, such as logic processors, graphics processors, etc. However, as the size of these devices decreases, so does the size and spacing (pitch) of the contact pads. This has necessitated devices which scale the small pitch of the discrete device contact pads up to a larger pitch for connection, for example to a printed circuit board (PCB). Such devices are referred to as interposers, and are well known in the art.
In addition, with this reduction in size comes a commensurate demand to pack greater numbers of discrete devices, such as memory, logic processor, graphics processor, etc. into a reduced volume. One known technique for reducing the volume into which discrete devices are packed is vertically stacking one device atop another, as opposed to laterally placing them on a printed circuit board. It is also known to use interposers to facilitate such stacking.
In one typical known embodiment, such as shown in FIG. 27, a die 150 (such as a logic processor) is connected on a first side 152 (e.g., front side) to an interposer 154. Bonding wires 156 electrically connect the contact pads of die 150 to contact pads on the first side of interposer 154. The pitch of the contact pads on interposer 154 is larger than that of die 150. Through-vias 158 in interposer 154 then deliver the electrical connection from the contact pads on first surface 152 to for example solder balls 160 second side (e.g., back-side) 162 of interposer 154 at the larger pitch of the contact pads on interposer 154. The structure may optionally be encapsulated in an appropriate insulative material 162 to protect the bond wires and electrical components. Solder balls 160 may then make connection, at a more appropriate or desired pitch, to a PCB, socket, etc. See, e.g., U.S. Pat. No. 5,783,870 entitled “Method of Connecting Packages of a Stacked Ball Array Structure”, which is incorporated herein by reference.
There are a number of drawbacks or challenges when using known interposers. First, interposers of the type described above require forming through-vias and filling those vias with conductive material. For silicon substrates, this etching typically done by a high-density plasma etch. This is a costly process, primarily due to the desired high density of filled vias. Other substrates may be employed in lower cost processes, although results are generally below the desired density. Low-density silicon feed troughs are available with wet etching. Glass-based interposers can be produced using bead blasting and conductive paste, but again at low density, and not on the desired silicon substrate. In general, there is a tradeoff between cost and desired density, as well as limits on the types of materials that may be used as a substrate for the interposer.
In addition, typical interposers are not flexible or compliant, in that they cannot accommodate relative movement between the stacked devices. This leads to stress-based failure of the electrical interconnections or structural integrity of the stacked structures. Furthermore, ball grid array (BGA) interconnections are relatively inflexible to out-of-plane connections. In the event that one or more of the solder balls are mis-sized or mis-placed, or a device is out of plane, they may either be incapable of making electrical interconnection or prevent other solder balls from making electrical interconnection between the interposer and the PCB, socket, etc.
Still further, optically transparent interposers with electrical feedthoughs are only available in low density, due in part to the limit on materials available for the interposer substrate and the methods of their fabrication.
Finally, once electrical connection to an interposer is made, typically by soldering or BGA, it is generally not reworkable. If the connection is inadequate, the interposer and die cannot be separated and reconnected. Rather, the two components are typically disposed of or recycled.
Therefore, there is a need in the art for an improved interposer that, inter alia, avoids costly via etching and filling, provides reliable, compliant connection between stacked devices, provides a high density of connections, and the option of an optically transparent substrate.